1. Field of the Invention
The invention relates to a new manufacturing process for aluminum-based structural elements containing at least two different alloys, by casting a plate or billet comprising at least two spatially separate alloys, followed by one or more hot transformation steps by rolling, extrusion or forging, and possibly one or several cold transformation steps, and intermediate and/or final heat treatments. The invention is particularly useful for manufacturing structural elements for aeronautical construction.
2. Description of Related Art
Parts with spatially variable mechanical characteristics are very attractive for mechanical construction. Traditionally, they are made by assembling two parts with different properties, but they are essentially homogeneous inside each part. The assembly can be mechanical (for example by bolting or riveting), or by gluing or any appropriate welding technique. Thus, bi-functional or multi-functional structural parts or elements can be obtained. This bi-functionalization or multi-functionalization may depend on the shape of assembled parts (which is not the meaning used herein) or may be related to their mechanical properties, particularly when two parts made of different alloys are assembled together. For example, transition joints are used in shipbuilding (see C. Vargel, Corrosion de l'aluminum [Corrosion of aluminum], Paris, 1998, Dunod, page 136), that are structural elements normally assembled by explosion welding starting from a steel part and an aluminum part. The steel side acts as a base onto which other steel parts are fixed, and the aluminum side acts as a base onto which other aluminum parts are fixed. Therefore these transition joints are bi-functional structural elements that avoid galvanic corrosion that will inevitably be set up in a damp environment between two dissimilar metals assembled traditionally.
Examples of multi-functional parts essentially made of aluminum arise for protection against corrosion and welding. Cladded plates can comprise a core protected on at least one side by an alloy skin with better resistance to corrosion and/or that is more easily meltable, either to protect the core against corrosion or to make it easier to weld to another part. Cladded plates are made by taking a preferably scalped alloy plate with a first composition (called the core alloy), and placing a thinner second and preferably scalped rolling ingot or a rolled plate (called the cladding alloy) onto the first plate. The cladded plate is then hot rolled to obtain a cladded strip, the hot rolling operation assuring a strong metallurgical bond between the two alloys. Cladded plates are monolithic parts, in the sense of the definition given below. They can be used in aeronautical construction, for example as a fuselage skin (for example, see U.S. Pat. No. 5,213,639 (Aluminum Company of America) or patent EP 1 170 118 (Pechiney Rhenalu). The cladding process can be used to fabricate large parts, but the chemical composition is variable through the thickness rather than over the length or width of the part. Thus, functionalization is fairly limited: the function of cladding is either protection against corrosion, or weldability.
In another approach to manufacturing of a monolithic bi-functional part, a different artificial aging treatment is applied to each end of a long product made of a single aluminum based alloy. Patent EP 0 630 986 (Pechiney Rhenalu) describes a process for manufacturing structurally hardened aluminum alloy sheets with a continuous variation of usage properties along a principal direction (length, width, thickness) of the product, in which final artificial aging is performed in a furnace with a special structure comprising a hot chamber and a cold chamber connected together through a heat pump. This process has been used to obtain small parts made of 7010 alloy about a meter long, in which one end is in the T651 temper and the other end is in the T7451 temper, by an isochronous artificial aging treatment. This process has never been developed industrially because it is difficult to control in a manner compatible with quality requirements in the aeronautical construction domain; these industrial difficulties increase with the size of the parts. Furthermore, if only one part made of a single alloy is used, the amplitude of the variation of mechanical properties along the length of the part is fairly limited. A significant improvement in this process is described in Application FR 2 868 084, but once again, the chemical composition of the alloy cannot be modified with this process.
A large variation in mechanical properties can be expected if two different aluminum alloys are used.
In the field of cast alloys, the manufacture of monolithic parts comprising several alloys is known in the art. WO 2005/063422 describes a process in which a semi-solid casting material made from unmixed stratified portions of alloys with sufficiently different solidification intervals is introduced into a permanent mold so as to essentially fill the same, and letting the casting material solidify therein.
Applicants are not aware of any industrially made monolithic worked parts comprising two alloys spatially separate from each other, made using a process other then cladding by hot rolling. The concept of starting from as-cast parts (for example extrusion billets, rolling ingots) comprising two spatially separated alloys is not new. A distinction between several approaches is made.
A first approach uses one or several fixed or mobile partitions. U.S. Pat. No. 3,353,934 (Reynolds) describes vertical casting of ingots or a billet with a vertical fixed partition, the partition running along the length of the slab. This fixed partition is made of marinite, stainless steel or graphite. The patent describes casting of alloy pairs 7075/6063, 7075/5052 and 7075/5083.
JP 48-005411 (Sumitomo) describes another vertical partitioning method applied to cast slabs. Another embodiment of casting with a vertical partition is described in patent application DE 44 20 697 (Institut für Verformungskunde and Hüttenmaschinen). U.S. Pat. No. 6,705,384 (Alcoa, Inc.) describes the use of one or more partitions in the form of a thin or thick aluminum plate that remains incorporated in the cast slab or billet.
Casting with partition has also been adapted to continuous casting between strips. Patents GB 1 174 764 and FR 1 505 826 (Glacier) describe the use of a mobile partition applied to casting between strips for casting of Al+6% Sn/AS5G alloy pairs.
A second approach uses the concept of an internal ingot mold; a first alloy is solidified in an internal ingot mold, and the solid shell thus formed acts as a mold for the second alloy. This concept is described in patent DE 844 806 (Wieland Werke). A metal tube or a hollow billet can also be used as an external shell in which a liquid alloy is cast as described in patent FR 1 516 456 (Kennecott Copper Corporation). This principle has been adapted to vertical continuous casting of cladded slabs in U.S. Pat. No. 4,567,936 (Kaiser). Patent application WO 2004/112992 (Alcan) describes several methods of forming rolling ingots comprising two alloys by semi-continuous vertical casting, using vertical separators. This process is particularly adapted for the fabrication of cladded rolling ingots.
All these processes according to the state of the art result in long cast products that contain two different alloys separated by partitions or interfaces parallel to the direction of casting.